1. Field of the Invention
The present invention relates to a test method of a mask for electron-beam exposure and a method of electron-beam exposure.
2. Description of the Related Art
An electron-beam exposure method of segmented projection type has been recently proposed as a novel method of electron-beam exposure to replace the cell projection method and the variable-shaped beam exposure method. This electron-beam exposure method of segmented projection type is a method wherein a prescribed primary pattern for projection is segmented into a plurality of divisions and every said division is subjected to exposure one by one till the whole of this prescribed primary pattern is transferred. Although the prescribed primary pattern is segmented into a plurality of divisions, this electron-beam exposure method of segmented projection type uses a mask or a pair of masks onto which the whole segmented portions of the prescribed pattern of one chip are formed in all. In this respect, the electron-beam exposure method of segmented projection type is altogether different from the variable-shaped beam exposure method wherein a pattern that is to be transferred is not actually formed onto the mask but processed as soft data or the cell projection method which employs a mask onto which only repeated parts of a prescribed pattern is formed. In consequence, this electron-beam exposure method of segmented projection type can markedly improve the throughput, compared with these conventional exposure methods.
This electron-beam exposure method of segmented projection type is explained well in the section of the prior art in Japanese Patent Application Laid-open No. 176720/1999 with reference to FIG. 2 in the publication. On the basis of this description, one example of the electron-beam exposure method of segmented projection type is described below.
FIG. 7 is a schematic view in explaining the electron-beam exposure method of segmented projection type. In FIG. 7, referential numeral 100 indicates a mask; 100a, a division on the mask 100; 100b, a demarcation region between divisions 100a; 110, a substrate, such as a wafer coated with a resist; 110a, a region for one die (one chip) on the substrate 110; 110b, a region for projection on the substrate 110, each corresponding to a division 100a; AX, an optical axis of an optical system of charged particle beam; EB, a charged particle beam and CO, a crossover point of the optical system of charged particle beam.
On the mask 100, being separated by a demarcation region 100b without a pattern, there are present numerous divisions 100a each of which is provided, on a membrane, a pattern to be transferred onto the substrate 110. Further, a support structure in the form of a grid is set over the demarcation region 100b, protecting the membrane thermally and mechanically. The mask 100 herein is a scattering membrane mask wherein, on a membrane, for example, a silicon nitride film with a thickness of 100 nm or so, there are formed electron-beam scatterer patterns made of, for example, tungsten with a thickness of 50 nm or so. This scattering membrane mask is the mask used mainly for the electron-beam exposure method of scattering-angle limiting type (referred to as xe2x80x9cSAL typexe2x80x9d hereinafter) and the exposure method herein is assumed to be the SAL type.
Every division 100a is provided with one of segmented patterns which the pattern that is to be transferred onto a region 110a for one die on the substrate 110 is segmented into, and every segmented pattern is transferred onto the substrate 110, one by one. The external appearance of the substrate 110 is as shown in FIG. 7(b). A section (the Va section of FIG. 7(b)) of the substrate 110 is shown in FIG. 7(a) on an enlarged scale.
In FIG. 7, the z-axis is taken parallel to the optical axis AX of the optical system of charged particle beam, and the x-axis and y-axis are taken parallel to the directions of the array of divisions 100a, respectively. While the mask 100 and the substrate 110 are moved continuously in opposite directions along the x-axis as arrows Fm and Fw indicate, respectively, patterns of divisions 100a in one line are transferred in succession through step-by-step scanning of the charged particle beam in the direction of the y-axis. After completing projection of the patterns in one line, divisions 100a in the next of that line in the direction of the x-axis receive scanning of the charged particle beam. Thereafter, in the same manner, projection (segmented projection) of divisions 100a is successively performed one by one so as to transfer the whole pattern for one die (chip).
The scanning order over the divisions 100a and the transcribing order onto the substrate 110 are presented by lines with arrowheads, Am and Aw, respectively. Hereat, the directions of movements for the mask 100 and the substrate 110 are opposite to each other, because the x-axis and y-axis for the mask 100 and the substrate 110 are reversed by a pair of projection lenses, respectively.
When the projection (segmented projection) is carried out in this manner, if patterns of divisions 100a in one line lying in the direction of the y-axis are projected on the substrate 110 by a pair of projection lenses as they are, gaps corresponding to the demarcation region 100b develop between regions for projection 110b on the substrate 110, each region for projection corresponding to a division 100a, respectively. To overcome this problem, the charged particle beam EB having passed through each division 100a is made deflected as much as the width Ly of the demarcation region 100b in the direction of the y-axis, whereby correction for the pattern projection position is made.
For the direction of the x-axis, besides moving the transmittable scattering mask 100 and the substrate 110 at respective specific speeds, in proportion to the ratio of pattern reduction, similar care is also taken. That is, when completing projection of divisions 100a in one line and turning to projection of divisions 100a in the next line, the charged particle beam EB is made deflected as much as the width Lx of the demarcation region 100b in the direction of the x-axis, whereby correction for the pattern projection position is made so as not to create a gap in the direction of the x-axis between regions for projection 110b. 
Although, in the mask in the above description of the segmented projection type method, the demarcation region to partition prescribed patterns is in the form of a grid, it can be stripe-shaped. In the case that such a mask is utilized, the projection of each division is carried out, while scanning electrically the inside of one zonal division partitioned by the stripe-shaped demarcation region, with the electron beam, in the direction of the length.
As described above, in the segmented projection type method, a mask or a pair of masks onto which the whole segmented portions of the prescribed pattern of one chip are formed in all are used so that the throughput thereof can be greatly improved as compared with the conventional cell projection method and the variable-shaped beam exposure method. Hereat, in the case that a plurality of masks onto which the whole segmented portions of the prescribed pattern of one chip are formed in all are utilized, the number of masks required is equivalent to the number of segmentation.
Further, in the segmented projection type method, since a support structure in the form of a grid can be set over the demarcation region 100b which is formed between respective divisions 100a, bending and thermal distortion of the mask substrate which may result from irradiation of the charged particle beam can be suppressed well and projection can be performed with high accuracy.
For the electron-beam exposure of segmented projection type described above, a mask (referred to as a xe2x80x9cscattering membrane maskxe2x80x9d, hereinafter) in which a pattern made of an electron-beam scatterer (for example, tungsten with a thickness of 50 nm or so) is formed on an electron-beam transmittable membrane with a relatively small electron-beam scattering power (for example, a silicon nitride film with a thickness of 100 nm or so) is employed and the SAL type pattern projection is conducted.
A schematic view of an ordinary SAL type optical system with a scattering membrane mask is shown in FIG. 8.
Herein the projection is made by an electron beam consisting of electrons (image-forming electrons) which are not scattered or scattered only with relatively small scattering angles, having transmitted a region of an electron-beam transmittable membrane 201a (referred to as a xe2x80x9cmembrane regionxe2x80x9d hereinafter) in a scattering membrane mask 201, on which no electron-beam scatterer 201b is formed. These image-forming electrons are focused by a first projection lens 202, and, after passing through a central opening in a limiting aperture section 203, are projected onto a resist 206 lying on a wafer 205 by a second projection lens 204. The resist 206 in the drawing is a negative one, of which an exposed portion is to remain, and shown in the form after the development for illustration. The resist can be a positive one. Meanwhile, electrons scattered with large scattering angles, having transmitted any of electron-beam scatterers 201b, are almost all cut off by a limiting aperture section 203 disposed in the cross-over position of the first projection lens or its vicinity. The image contrast is, in this way, formed on the wafer, owing to the difference in electron-beam scattering angle between the electron-beam scatterer region and the membrane region, which forms the pattern.
A description of a manufacturing method of a scattering membrane mask can be found, for example, in SPIE, Vol. 3236, pp. 190 (1998). An example of a manufacturing method of a scattering membrane mask is described below.
First, upon a silicon substrate, silicon nitride films are formed as electron-beam transmittable thin films (membranes) by the LPCVD (Low Pressure Chemical Vapour Deposition) method. The silicon nitride films are hereat, formed on the both surfaces of the silicon substrate. Subsequently, by means of the sputtering, a thin chromium film is formed as an etching stopper layer over the silicon nitride film lying on the top surface of the substrate, and thereon a tungsten layer is grown as a scatterer layer.
Next, over the silicon nitride film formed on the backside of the silicon substrate, a coating of a resist is applied and patterned, and using the formed resist pattern as a mask, the silicon nitride film is removed by reactive ion etching so as to expose the silicon substrate in a prescribed region. The tungsten layer can be formed, after this step, over the silicon nitride film lying on the top surface of the substrate.
After the removal of the resist, by carrying out wet-etching with KOH or dry etching, silicon in the exposed region of the silicon substrate is removed and thereby an opening section to expose the silicon nitride film formed on the top surface of the substrate is formed.
Thereat, over the tungsten layer lying on the top side of the substrate, a coating of a resist is applied and patterned, and using the formed resist pattern as a mask, the tungsten layer is patterned by dry etching. By removing the resist, a scattering membrane mask in which a tungsten layer pattern is formed on the silicon nitride film is accomplished. The exposed portion of the thin chromium film is also removed by means of wet etching.
Since the scattering membrane mask described above is a mask in which a pattern is formed through growing an electron-beam scatterer on a very thin electron-beam transmittable membrane of 100 nm or so, its fabrication is generally a difficult task and a problem of a dispersion in the film thickness of the electron-beam transmittable membrane is particularly liable to happen. The dispersion of the thickness arising in the membrane region (the region the electron-beam transmittable membrane alone occupies) causes a dispersion in the irradiation intensity of image-forming electrons which irradiate the resist on the wafer and leads to deterioration of linewidth accuracy of the pattern. That is, when some electrons transmit thicker parts of the membrane region, the scattering angles of those electrons become larger, which increases the number of electrons cut off by a limiting aperture section and decreases the number of electrons to irradiate the resist on the wafer so that the corresponding irradiation intensity is lowered. This results in thinning of the pattern, if the resist utilized is a negative one.
The thickness of the electron-beam transmittable membrane in the scattering membrane mask is, hitherto, measured during its manufacturing step. For example, at the time when deposition of an electron-beam transmittable membrane material such as SiN on a silicon substrate is completed, its thickness is measured by a known method. In such a measurement, the thickness of the membrane region in the finished mask cannot be obtained. Moreover, as the measurement of the film thickness is made prior to pattern formation, a film thickness distribution of the membrane regions on the pattern region surface of the finished mask cannot be known, either. Further, even if tried, with any ordinary conventional measuring method, the measurement of the thickness of the membrane region in the finished mask is considerably difficult, because the membrane region which is the very measuring object is partly situated between minute scatterer patterns, and besides the silicon substrate is absent on the backside of the membrane region (the open space is present there).
In Japanese Patent Application Laid-open No. 258703/1993, there are disclosed an electron-beam test method wherein, with a charged particle beam being sent forth, the surface of a substrate is scanned and, at least, charged particles of one type among three types that come out from either the top face or the bottom face of that substrate, namely, secondary charged particles, back-scattered charged particles and transmitted charged particles, are detected and a system thereof. This method and system are described to be applicable to tests of an X-ray mask, a mask for electron-beam and a stencil mask, and able to detect defects of various types as well as distinguish those defects. With respect to the measurement of the film thickness of the substrate, however, nothing is mentioned therein.
An object of the present invention is to provide a test method of a scattering membrane mask for electron-beam exposure wherein, even after mask fabrication, a film thickness distribution of membrane regions thereof can be readily measured as well as a method of electron-beam exposure wherein a pattern can be formed with high linewidth accuracy.
The present invention relates to a test method of a mask for electron-beam exposure which has a pattern region in which, by forming an electron-beam scatterer in prescribed shape on an electron-beam transmittable thin film, a scattering region with said electron-beam scatterer and a membrane region without said electron-beam scatterer are formed in prescribed pattern shape; wherein
electron-beam irradiation onto a tested mask is carried out in a plurality of times, with each irradiated region subjected to irradiation at a time being scanned with the electron beam, and through detection of transmitted electrons which is made for each irradiated region subjected to irradiation at a time, the value for ITE/xcex1 given by dividing a cumulative intensity of the transmitted electrons ITE for an irradiated region subjected to irradiation at a time by a pattern density xcex1 of a membrane region within said irradiated region is obtained for each one of a plurality of irradiated regions, whereby data of a relative film thickness distribution for the membrane regions in the mask face are acquired.
Further, the present invention relates to the test method of a mask for electron-beam exposure as set forth above, wherein an electron-beam transmittance ITE/(xcex1xc2x7IIE) for each irradiated region subjected to irradiation at a time is obtained from a cumulative intensity of the transmitted electrons ITE for an irradiated region subjected to irradiation at a time, a pattern density a of a membrane region within said irradiated region and a cumulative intensity of the irradiating electron beam IIE applied onto said irradiated region; and, using the relationship between the electron-beam transmittance and the normalized mask thickness tnor that is expressed by Equation (1), together with each obtained electron-beam transmittance ITE/(xcex1xc2x7IIE), the corresponding average normalized mask thickness of a membrane region for an irradiated region subjected to irradiation at a time is made out for each one of a plurality of irradiated regions.
tnor=tm/RGxe2x80x83xe2x80x83(1)
where tm is the thickness of the mask (the membrane region) and RG is the Grxc3xcn range.
Further, the present invention relates to the test method of a mask for electron-beam exposure as set forth above, wherein the average film thickness of a membrane region of each irradiated region subjected to irradiation at a time is made out by multiplying the average normalized mask thickness of a membrane region obtained for each irradiated region subjected to irradiation at a time by the Grxc3xcn range, whereby data of a film thickness distribution for the membrane regions in the mask face are acquired.
Further, the present invention relates to the test method of a mask for electron-beam exposure as set forth above, wherein the average film thickness of a membrane region of each irradiated region subjected to irradiation at a time is made out from the average normalized mask thickness of a membrane region obtained for each irradiated region subjected to irradiation at a time, using Equation (1) and Equation (2), whereby data of a film thickness distribution for the membrane regions in the mask face are acquired.
RG=(4.0xc3x9710xe2x88x922/xcfx81)Vacc1.75[xcexcm]xe2x80x83xe2x80x83(2)
where xcfx81 is the density of the mask material in g/cm3 and Vacc is the accelerating voltage in kV.
Further, the present invention relates to the test method of a mask for electron-beam exposure as set forth above, wherein a converging lens that makes the transmitted electrons having transmitted a membrane region of the tested mask converge is placed and besides, in the cross-over position of said converging lens or its vicinity, a limiting aperture section is disposed, and, through the detection of the transmitted electrons passing through the central opening in said limiting aperture section, a cumulative intensity of the transmitted electrons ITE for each irradiated region subjected to irradiation at a time is obtained.
Further, the present invention relates to a scattering-angle limiting type electron-beam exposure method of segmented projection type using a mask which has a pattern region in which, by forming an electron-beam scatterer in prescribed shape on an electron-beam transmittable thin film, a scattering region with said electron-beam scatterer and a membrane region without said electron-beam scatterer are formed in prescribed pattern shape, and on which a prescribed primary pattern that is to be transferred for one chip or a portion of several segments thereof is formed, being segmented into a plurality of divisions; wherein
said mask is tested in any test method of the present invention as set forth above and, thereat, electron-beam irradiation is carried out with each division of said mask functioning as an irradiated region subjected to irradiation at a time, and thereby an average film thickness or normalized mask thickness of a membrane region in each division is obtained; and
from the average film thickness or normalized mask thickness obtained for each division of said mask, the corresponding aperture transmission of the membrane region of the division is made out through the use of the relationship between the film thickness and the aperture transmission which is obtained beforehand, setting the aperture angle at that of a limiting aperture section in an electron-beam exposure system that is to be used and the accelerating voltage at that for making the electron-beam exposure; and
from the aperture transmission obtained for each division of said mask, a varying degree of the electron-beam exposure time for applying irradiation to each division of the mask is evaluated so that an identical amount of exposure dose may be given to each region on a substrate, each corresponding to a division of the mask; and
in accordance with the varying degree of the electron-beam exposure time obtained for each division of said mask, the electron-beam exposure is applied to each division of the mask one by one, whereby the prescribed primary pattern is transferred.
Further, the present invention relates to the electron-beam exposure method as set forth above, wherein, from the pattern density a obtained for each division of the mask, a corresponding extent of refocusing for each division is determined using the relationship between the beam current and the defocus length which is measured beforehand on a substrate in an electron-beam exposure system that is to be used, and while correcting the focus in accordance with the extent of refocusing obtained for each division, the electron-beam exposure is applied to each division of the mask one by one, whereby the prescribed primary pattern is transferred.
In a test method of the present invention, even after mask fabrication, data concerning a film thickness distribution for the membrane regions within a mask face can be readily obtained without making any destructive tests. Further, the use of the pattern density obtained through detection of secondary electrons that is carried out concurrently with detection of transmitted electrons and the subsequent image processing thereof makes it possible to perform a film thickness inspection with a higher accuracy. Moreover, a test of ordinary pattern defects can be made at the same time by the detection of secondary electrons and the image processing so that a mask test can be conducted with great efficiency.
Further, in a method of electron-beam exposure of the present invention, measurements of a scattering membrane mask that is to be used are made by a test method of the present invention and data concerning the film thickness of a membrane region for each division are collected, and on the basis of these obtained data, the projection condition is determined, and, under that projection condition, the projection is carried out. In this way, the pattern formation can be carried out with high linewidth accuracy, regardless of a dispersion of the mask thickness.